This invention relates to electrochemical cells. More particularly, it is concerned with high power primary electrochemical cells having an oxidizable anode, for example lithium, and a reducible liquid cathode material, for example thionyl chloride.
Electrochemical cells which employ soluble or liquid cathode materials have undergone rapid development in recent years. In these cells the active cathode material is a fluid solvent and the active anode material is a highly electropositive metal, such as lithium. An electrolyte solute is dissolved in the solvent cathode material to provide electrical conductivity. During discharge the solvent is electrochemically reduced on a cathode current collector to yield ions from the anode to form insoluble metal salts, e.g., metal halides.
One particular type of electrochemical cell of the foregoing class contains an anode of lithium and a reducible liquid cathode of thionyl chloride. Typically the electrolyte solute dissolved in the solvent is lithium tetrachloroaluminate. These electrochemical cells have proven to have outstanding weight and volume energy density. The lithium anode and the cathode current collector are relatively thin, less than 1 and 2 mm, respectively. Thus, these cells have a high electrode surface to volume ratio and, therefore, very high power capability.
When such a cell becomes short circuited, either internally or externally, very high short circuit currents flow through the cell. These currents cause heating of the components within the cell which may be sufficient to cause melting of some of the materials. Lithium melts at about 180.degree. C. Molten lithium may react with the thionyl chloride or with the discharge products produced within the cell in a violent chemical reaction with possible undesirable consequences.
A number of approaches have been proposed to resolve the problems which occur when lithium batteries become overheated during abnormal high rate operation as under short circuit conditions. Pressure sensitive vents which open to the atmosphere when the vapor pressure of the electrolyte builds up with increasing cell temperature have been investigated. Current sensitive and thermally sensitive fuses have been installed within the cell. Other techniques which have been employed include fabricating the lithium anode as an alloy with other materials and also altering the concentration of the solute in the electrolytic solution. While these various techniques assist in solving the problem for various situations, under certain conditions they are either difficult to employ or introduce additional problems.